Hercules is located at the northerly extent of the Berkeley Hills range of the Coastal Province of California.
Refugio Valley is formed by two low ridges of hills descending about four miles northwest from elevations of about 800 feet to nearly sea level at the shore of San Pablo Bay.
Refugio Creek’s watershed - the Refugio Valley - lies at the northern extent of a bed of shale rock that makes up the Berkeley Hills. Almost all of the surface soil in the Refugio watershed is this shale rock. Small outcrops of other rock types are found along valley bottoms and the San Pablo Bay waterfront.
In the upper Refugio Valley the shale soil layers are thick and generally stable. Beneath the surface of lower Refugio Valley, below the old shoreline of San Pablo Bay, are thinner layers of alluvium resting on deposits of bay muds.